The present invention relates generally to a fan blade used in turbine engine applications composed of two or more components made from different materials, and more particularly to a formulation used to promote adhesion between the different materials in the construction of a lightweight jet engine fan blade.
Gas turbines include, but are not limited to, gas turbine power generation equipment and gas turbine aircraft engines. A gas turbine includes a core engine having a compressor to compress the air flow entering the core engine, a combustor in which fuel is burned by mixing it with compressed air to generate a propulsive gas flow, and a turbine which is rotated by the propulsive gas flow. The turbine is connected by a shaft to drive the compressor. In a typical gas turbine used in aircraft engine applications, the compressor portion is comprised of a low pressure compressor located in the front of the engine and a high pressure compressor located aft of the low pressure compressor and fore of the combustor section of the engine; while the turbine portion is comprised of a high pressure turbine located immediately aft of the combustor section and a low pressure turbine located aft of the high pressure turbine. The low pressure turbine is typically connected by a smaller diameter coaxial shaft to drive a fan located forward of the compressor section and to drive the low pressure compressor. The low pressure compressor sometimes is called a booster compressor or simply a booster and is optional. The high pressure turbine drives the high pressure compressor and provides auxiliary power to the engine and plane.
The compressors and the turbines have rotating portions and stationary portions. Typically, the rotating portions of the turbine extract some energy from the gases of the combusted fuel to provide energy to rotate the rotating portions of the compressor and fan and to supply other energy needs of the plane. The remaining energy from the gases provides the thrust to drive the engine. The stationary portions of the turbine and the compressor, frequently referred to as stators, direct the gases flowing through the engine. The fan, the compressor and the turbine portions of the engine are comprised of airfoils. The airfoils in the rotating portions of the engine are frequently referred to as blades and are attached to rotors or discs, while airfoils in the stator portions of the engine are referred to as vanes and are attached to casings or housings. Airfoils each including an airfoil portion attached to a shank portion which provide attachment to the associated structure, disks or casings. Typically, there are alternating circumferential rows of radially-outwardly extending rotor blades and radially-inwardly extending stator vanes. When present, a first and/or last row of stator vanes (also called inlet and outlet guide vanes) may have their radially-inward ends also attached to a non-rotating gas turbine stator casing. Counterrotating xe2x80x9cstatorxe2x80x9d vanes are also known.
Conventional airfoil designs used in the compressor section at the engine typically have airfoil portions that are made entirely of metal, such as titanium, or are made of a composite such as the GE-90 fan blade used on the Boeing 777. A xe2x80x9ccompositexe2x80x9d is defined to be a material having any (metal or non-metal) fiber filament embedded in any (metal or non-metal) matrix binder, but the term xe2x80x9ccompositexe2x80x9d does not include a metal fiber embedded in a metal matrix. The term xe2x80x9cmetalxe2x80x9d includes metal alloys such as titanium Alloy 6-2-4-2. An example of a composite is a material having graphite filaments embedded in an epoxy resin.
The all-metal blades, including costly wide-chord hollow blades, are heavier in weight which results in lower fuel performance and require sturdier blade attachments, while the lighter all-composite blades are more susceptible to damage from ingestion events. Known hybrid blades include a composite blade having an airfoil shape which is covered by surface cladding (with only the blade tip and the leading and trailing edge portions of the surface cladding comprising a metal) for improved resistance to erosion and foreign object impact. The fan blades typically are the largest (and therefore the heaviest) blades in a gas turbine aircraft engine, and the front fan blades are usually the first to be impacted by foreign objects such as birds. Various designs are under consideration for gas turbine blades having reduced weight for use as gas turbine fan blades that are comprised of metal and non-metal materials and have the capability to resist damage from ingestion of foreign objects. Some of these designs include lightweight inserts molded into cavities of metal blades. The cavities are regions of the blade that have had metal removed to lighten the blade, and the lightweight inserts are added to restore an aerodynamic shape to the blade that was altered by the inclusion of the cavity in the blade design. While both the lightweight inserts and the metallic portion of the blade are relatively monolithic materials having excellent strength, the interface between the lightweight inserts and the metallic portion of the blade is the weak link in which debonding and failure is most likely to occur. While failures which result in the separation of inserts from the blade are not catastrophic, they are undesirable as they will cause an aerodynamic inefficiency loss as well as an imbalance condition in the fan. What is needed are bonding materials and methods that promote adhesion between the metal portion of the blade and lightweight inserts and improve the adhesion of the lightweight inserts molded into blade pockets over the life of the blade.
The present invention is directed to improvements in adhesion between the metallic portion of a fan blade and lightweight inserts that are added to a fan blade to improve the aerodynamic flow of air over the blade. The fan blade is manufactured to have a lighter weight by removing metal at preselected locations. These preselected locations take the form of pockets. The locations of the pockets are preselected so as not to adversely affect the structural integrity of the blade. However, the pockets do adversely impact the aerodynamic flow of air over the blade. The aerodynamic flow is restored by filling the pockets with the lightweight inserts. Overall, the final blade retains its structural integrity that is, its resistance to foreign object impacts and the like and its aerodynamic flow, even though its weight has been reduced.
The formulation of the lightweight insert comprises an elastomer composition having an optional anti-oxidant, an optional ultraviolet absorber (UVA) and an optional hindered amine light stabilizer (HALS). The elastomer is formed by mixing a curative with a prepolymer and casting the resulting mixture into a mold that includes the pockets formed in the metallic blade. The specific composition of the elastomer is the subject of a copending, related application, 13DV-13058, assigned to the assignee of the present invention and incorporated herein by reference.
Although both the insert, comprised of cured elastomer, and the metallic blade are monolithic materials having good strength, the interface between the metallic blade and the elastomeric insert, that is, the metallic pockets and the faces of the elastomeric insert that contact the metallic pockets, tends to be the weak point in the system, even though the elastomer may form a strong bond with the metallic blade. As a result of the centrifugal forces resulting from the high speed rotation of the fan blade and any damage to the inserts resulting from foreign object impact that may weaken the bond, failure of the blade, which is defined herein to be the separation of the elastomeric insert from the metallic blade, most frequently occurs at this interface. The solution to the problem is to strengthen the bond between the insert and the metallic pockets to the blade.
One solution to this problem is to apply a chemical formulation to the blade that promotes improved bonding between the metallic blade and the elastomeric insert. The chemical formulation can be applied to the blade or to the elastomeric insert. However, the adhesion at the interface between the chemical formulation and the metallic blade and between the chemical and the elastomeric insert must be stronger than the adhesion between the elastomeric insert and the metallic blade. Even though the interface formed by the chemical formulation between the blade and the insert may still be the weak link in the system, any improvement in strength at the interface will extend the life of the blade by increasing the mean time between failures. As used herein, the chemical formulation applied to improve bonding between a metallic blade and an elastomeric insert is referred to as an adhesion enhancer.
Another solution to this problem is to roughen the pockets of the blade so that a mechanical bond can be formed between the elastomeric insert and the metallic blade. This bond provides additional strength in addition to the chemical bond that is formed between the insert and the metallic portion of the blade. An additional benefit resulting from this surface roughness is that the surface area available for chemical bonding of the insert to the blade is also enhanced.
An advantage of the present invention is that the interface between the metallic portion of blade and the elastomeric insert can be made stronger and more reliable, thereby extending the life of the blade.
Another advantage of the present invention is the improved bonding between the metallic portion of the blade and the elastomeric insert can be accomplished with a minimal amount of effort, by either appropriate application of a chemical that promotes improved adhesion or by manufacturing a blade that has an interface surface that can include both components of chemical bonding and improved mechanical bonding. Of course, a combination of a chemical that improves adhesion between an elastomeric insert and a blade having improved mechanical bonding capabilities can also be utilized.
Another advantage of the present invention is that the elastomer can still be cured directly to the blade, regardless of the type of adhesion enhancer that is selected. Because the pockets form part of the mold, the elastomer will mate with a preapplied chemical that promotes adhesion, or with an interface surface area of the blade that has been roughened. Of course, both the adhesion enhancer and the roughened blade result in no misfit between the pocket and the blade, so that the blade having the cured elastomeric insert is aerodynamic, with little or no trimming required to remove excess material. This permits unimpeded flow of air entering the compressor while allowing the blade to operate at temperatures up to 310xc2x0 F. (155xc2x0 C.).
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.